This invention relates to a method for analyzing the concentration of oxygen in various gases, particularly to a method for analyzing trace (sub-ppb) levels of oxygen concentration.
A known type of oxygen concentration analyzer is such that a probe sensor, when supplied with a gas to be analyzed, produces an electrical output which is proportional to the concentration of oxygen in the gas. In a version of this type, oxygen in the gas reacts with water in the electrolyte on the surface of an electrode in the probe sensor to generate hydroxyl ions which migrate to the other electrode across the electrolyte, whereupon an electric current flows in a quantity proportional to the oxygen concentration in the gas. This operating principle is used by a Galvanic cell type oxygen concentration analyzer.
A conventional type of oxygen concentration analyzer which produces a current or other electrical output proportional to the concentration of oxygen in the gas to be analyzed is shown schematically in block form in FIG. 4. The analyzer has two gas feed lines, one of which is supplied with an oxygen-free gas (hereunder referred to as a zero gas) and the other line with a gas of a known oxygen concentration (which is hereunder referred to as a standard gas) and a sample gas. The sample gas and the standard gas are selectively supplied at separate lines via switch valves 1 and 2. The feed line for the zero gas has a switch valve 3 and the feed line for the sample gas or standard gas has a switch valve 4 and by selectively opening these valves, either the zero gas or the sample or standard gas will be fed to a flow regulator which controls the gas to flow to the probe sensor at optimum rate.
The analyzer contemplated by the invention determines the oxygen concentration using a calibration curve according to the following principle. The calibration curve is a straight line on a graph that represents a proportional expression describing the relationship between the oxygen concentration in a gas and a current or other electrical output of the probe sensor. An example of the calibration curve is shown in FIG. 2. The variable x on the horizontal axis represents the oxygen concentration in the sample gas, the variable y on the vertical axis represents the sensor output, a represents the slope of the calibration curve, and b represents the height at which the calibration curve intercepts the vertical axis. The first step of measurement is to determine the mathematical expression for the calibration curve, y=ax+b. To this end, the zero gas is first passed into the sensor and the sensor output (b) is determined (since the sensitivity of the sensor varies with ambient temperature and other factors, the output (y=b) for the zero gas (x=0) does not necessarily register zero). Then, the standard gas is passed into the sensor to determine the output (y1) for a known oxygen concentration (x1). Suppose here that b is 100 μA, x1 is 10 ppb and y1 is 200 μA. Substituting these values into y=ax+b, we obtain 200=10a+100. Since a=10, the calibration curve is expressed by y=10x+100.
In the next step, a sample gas of an unknown oxygen concentration is passed into the sensor. If the sensor output (y) is 150 μA, 150=10x+100, so the oxygen concentration (x) in the sample gas is determined as 5 ppb.
The actual measurement of oxygen concentration starts with passing the zero gas into the sensor which produces an output to be represented on the display in the analyzer. If the sensor output is not zero (if b is 100 μA as in the illustrated case), the correction dial is rotated so that the sensor output reads zero (this correction is hereunder referred to as zero adjustment and as the result of zero adjustment, the calibration curve can be expressed by y=ax). In the next step, the standard gas is passed into the sensor and the output y is determined. Since x is a known value, the slope of the calibration curve (a) is determined and the concentration span is adjusted. To be more specific, an adjustment is made such that the sensor output takes a specified value that corresponds to the known oxygen concentration in the standard gas. Subsequently, the sample gas is passed into the sensor and the sensor output can be converted to the oxygen concentration in the sample gas.
Thus, in the prior art method, the zero gas, the standard gas and the sample gas are sequentially passed into the sensor by switching one gas to another. If the sensor output has not completely stabilized, zero adjustment has to be made. Even if the sensor output completely stabilizes after zero adjustment, it may occasionally vary due to ambient temperature or other factors, potentially causing the display in the analyzer to register a negative output for the sample gas. The sensor output for the sample gas can also drop if the sensor performance deteriorates with time on account, for example, of a drop in reactivity on the electrode surface in the sensor or of a drop in ion mobility due to stained electrolyte. The effects of ambient temperature and other factors are particularly significant when trace (sub-ppb) levels of oxygen concentration are to be measured with a Galvanic cell type analyzer.
In order to solve these problems, the gas to be flowed into the sensor is frequently switched from the sample gas to the zero gas and zero adjustment is made. Of course, the sample gas cannot be analyzed as long as zero adjustment is performed but this is not desirable in facilities that perform continuous measurement of trace (sub-ppb) levels of oxygen concentration (e.g. a gas control facility which performs continuous analysis and monitoring of the oxygen concentration in the exit gas from a high-purity gas refinery or a facility which performs continuous analysis and monitoring of the oxygen concentration in an inert gas feed gas and a carrier gas to a semiconductor fabrication plant) because any sudden change in oxygen concentration that occurs in these facilities must be measured without time delay. As a further problem, when the sample gas is switched over to the zero gas to start zero adjustment, not only the zero gas but also the sample gas in a sump on the gas feed line tends to be introduced into the sensor; as a result, the time required to stabilize the zero point is prolonged, making it difficult to perform efficient zero adjustments.